Distortion detection for a power amplifier

ABSTRACT

The input and output of amplifier ( 110 ) are sampled and downconverted in frequency and filtered at ( 124  and  126 ) to produce sub-band signals. The input signal sub-band ( 134 ) is then subtracted from the output signal sub-band ( 136 ) at ( 130 ) to produce a residual signal ( 132 ) containing any distortion present in the sub-band selected by the downconversion oscillator ( 128 ). Signal analyser ( 158 ) compares the input and output signal energy to deduce the amount of distortion in the output signal, to provide signals for controlling a distortion reduction mechanism such as a predistorter operating on amplifier ( 110 ). Alternatively, the input signal sub-band ( 134 ) and the output signal sub-band ( 136 ) are Fourier transformed ( 210  and  212 ) to produce spectrums which can be analysed to determine the presence of distortion in the output of amplifier ( 110 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatus for detecting distortionin an output signal which a signal handling means (such as an amplifier)produces in response to an input signal.

2. Description of the Related Art

It is known to use distortion reduction mechanisms on amplifiers in anattempt to linearise the amplifier's response. It is often desirable tomonitor residual distortion in the output of the amplifier, and to usethis signal to adapt the distortion reduction mechanism to maximise thecancellation of the distortion. It is known to use a pilot signal todetect residual distortion. The pilot signal is added to the inputsignal and detected in the output signal so that any distortion itexperiences can be assessed. The use of pilot signals for distortiondetection has the disadvantage that a pilot signal must be generated,detected and then removed from the output signal afterwards. Thus, theuse of pilot signals increases system complexity and adds the risk thatspurious noise or continuous wave (CW) emissions introduced by the pilotsignal detection system will remain as unwanted residues in theamplifier output.

SUMMARY OF THE INVENTION

An object of the invention is to provide the detection of distortionwithout the use of pilot signals and ameliorate at least some of theproblems identified above.

According to one aspect, the invention provides a method of detectingdistortion in an output signal which a signal handling means produces inresponse to an input signal, the method comprising isolating as a firstsignal a frequency range of the input signal and in parallel and as asecond signal a frequency range of the output signal, and comparing thefirst and second signals to assess the output signal for phasedistortion.

According to another aspect, the invention provides apparatus fordetecting distortion in an output signal which a signal handling meansproduces in response to an input signal, the apparatus comprising meansfor isolating as a first signal a frequency range of the input signaland in parallel and as a second signal a frequency range of the outputsignal, and comparing means for comparing the first and second signalsto assess the output signal for phase distortion.

By comparing the input and output signals in this way, the presence ofphase distortion in the output signal can be assessed, and a pilotsignal is not used.

In the preferred embodiment, the frequency ranges are substantially thesame, and the isolating step may involve frequency converting the inputand output signal to produce the first and second signals. Preferably,the frequency conversion is frequency down-conversion which means thatthe resulting signals are of lower frequency and therefore provide lessdemands on the devices used to handle them. Advantageously, the samesignal can be used to frequency down-convert both the input and outputsignals.

The isolating step may be performed at a series of different frequencyranges. The series may be such that at least two adjacent ranges overlapor abut one another. Alternatively, it can be arranged that at least twoof the ranges are discontinuous, so that complete frequency coverage isnot provided. In one embodiment, the isolating step traverses the seriesof ranges in a sequence which is not in frequency order. This may helpto avoid interactions between the temporal characteristics of the inputsignal and the isolating step sequence.

Frequency ranges used in the isolation step may be fixed or chosen tocover a frequency range of interest (for example, to target an activepart of the frequency band of the input signal). In a preferredembodiment, at least one frequency range used in the isolation step canbe chosen dynamically.

Advantageously, the comparing step comprises subtracting the fist signalfrom the second signal to produce a residual signal. The residual signalmay contain substantially only residual distortion that has been causedby the signal handling means. The signals involved in the subtractionmay need to be scaled relative to one another so that the wanted signals(i.e. those which are ideally to be obtained from the signal handlingmeans) cancel leaving only the distortion which it is desired to detect.Such scaling of the signals may be needed where the signal handlingmeans amplifies the input signal.

The comparison step may involve the correlation of the residual signalwith a reference distortion signal to produce a signal indicative ofdistortion in the residual signal corresponding to the referencedistortion signal. Preferably, the reference distortion signal isderived from the first signal (which is part of the input signal). Thereference distortion signal may be derived by multiplying the firstsignal with itself repeatedly.

The comparison step may involve transforming the residual signal to thefrequency domain for further analysis. In this way, the spectrum of theresidual distortion can be analysed. Preferably, the first signal isalso transformed to the frequency domain (e.g. by FFT), thus allowingcomparisons to be made between the two spectra. For example, thespectrum of the first signal may be used to identify any remainingwanted signal components which have not been removed from the residualdistortion signal or to provide a reference against which the magnitudeof components in the residual distortion spectrum can be measured.

In a particularly preferred embodiment, the inventive method andapparatus for detecting distortion is used to control a distortionreduction mechanism operating on a distorting signal handling means. Inthis way, the distortion reduction mechanism may be adapted to maximisethe cancellation of the distortion created by the signal handling means.For example, the distortion reduction mechanism could be a predistortionlineariser or a feed-forward lineariser.

By way of example only, certain embodiments of the invention will now bedescribed with reference to the accompanying figures, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 each schematically illustrate distortion detectingapparatus applied to a non-linear amplifier.

DETAILED DESCRIPTION

Distortion detection scheme 100 in FIG. 1 operates on non-linearamplifier 110. Amplifier 110 is a wide band radio frequency poweramplifier having a non-linear characteristic such that it imposesintermodulation distortion upon signals passing through it. Thedistortion detection scheme 100 provides signals indicative of thisdistortion which can be used by an appropriate distortion reductionmechanism (not shown), for example, a predistorter operating on theinput 112 to amplifier 110, subsequent to coupler 114.

The input signal 112 is sampled at coupler 114 along path 116. Theoutput 118 of the amplifier 110 is sampled at coupler 120 along path122. The signals on paths 116 and 122 are frequency down-converted andfiltered by Down Conversion, Filtering and Digitisation (DCFD) units 124and 126 respectively. The downconversion performed by units 124 and 126is performed by mixing the incoming signal with a local oscillator (LO)signal from frequency controlled synthesiser 128. The digital output ofDCFD 126 therefore corresponds to a coincident sample of a sub-band ofoutput signal 118. Similarly, the digital output of DCFD 124 correspondsto a sample of the same sub-band of input signal 112.

It will be appreciated that DCFD units 124 and 126 can be adjusted topass a different sub-band from the input signal 112 and the amplifieroutput signal 118 respectively. This can be achieved by adjusting thefrequency of the LO signal from synthesiser 128 so that differentfrequency ranges are mixed down to the filter passbands within DCFDunits 124 and 126. Hence, the frequency band of the input and outputsignals can be divided into a number of sub-bands which can be evaluatedin turn by adjusting the frequency of the output of synthesiser 128. Thedesired sub-bands could be overlapping or abutting so as to cover thewhole frequency band of interest but, on the other hand, it might bethat they do not cover the whole of the frequency band of the input andoutput signals in a contiguous fashion. It is possible for the desiredsub-bands to be assigned dynamically by, for example, detecting onlythose sub-bands corresponding to portions of the frequency band of theinput and output signals which contain signal energy at a particulartime.

Since it is the distortion present in signal 118 which is of interest,the detection scheme 100 forms a signal indicative of this distortion.This is achieved by using subtractor 130 to produce a residual signal132. Subtractor 130 subtracts the input signal sub-band 134 provided byDCFD 124 from the output signal sub-band 136 provided by DCFD unit 126.This subtraction process cancels components in sub-band 136 whichcorrespond to actual components of the input signal 112 to amplifier110. The residual signal 132 therefore contains substantially onlycomponents of sub-band 136 which correspond to distortion present in theoutput 118 of amplifier 110. To ensure the cancellation of components ofinput signal 112 in sub-band 136, sub-band 134 is scaled by scaler 138which matches the amplitude of components due to input signal 112 in thetwo sub-band signals 134 and 136 (the signal 118 being an amplifiedversion of signal 112).

The power of the residual signal 132 is then measured using a powerdetector 148 to determine the amount of distortion in the output 118 ofamplifier 110 within a given sub-band. A complete picture of distortioncontained in the amplifier output 118 can then be acquired by steppingthrough the sub-bands by changing the frequency of the LO signalproduced by synthesiser 128. Power detector 150 measures the powerpresent within a given sub-band of the input signal 112.

The pattern of distortion energies measured in the various sub-bandsacross the frequency band of interest and the corresponding pattern ofinput signal energies are then acquired by the signal analyser 158. Thesignal analyser 158 can determine the amount of third-order and otherorders of distortion present in the amplifier output signal 118 basedupon the acquired data. The signal analyser's measurements 159 ofthird-order and higher order distortion components can then be used tocontrol an amplifier linearisation mechanism, for example, apredistorter or a feed-forward linearisation system.

It will be appreciated that many of the signal processing functionsillustrated in the block diagrams of FIG. 1 and FIG. 2, including partsof the DCFD units and the synthesiser, can be implemented digitally aswell as by using analogue techniques. It will also be appreciated thatthe invention apparatus is particularly suited to integration, sincebreaking the frequency band of interest into narrower sub-bands allowsthe use of integrated analogue to digital converters and digital signalprocessing techniques.

The distortion detection scheme 200 shown in FIG. 2 is moresophisticated than scheme 100 and can advantageously use similarhardware as was used in scheme 100, (and therefore keep costssubstantially the same) but it can exploit more powerful signalprocessing techniques to analyse the distortion products contained inthe amplifier output 118. The distortion detection scheme 200 of FIG. 2is similar in many ways to detection scheme 100. Therefore, elements ofdetection scheme 200 already described with reference to detectionscheme 100 retain the same reference numerals and will not be describedagain in detail in the following discussion.

In detection scheme 200, the output signal sub-band 136 and the inputsignal sub-band 134 are each translated from the time domain to thefrequency domain by, for example, a digital Fourier transformationprocess, indicated by FFT units 210 and 212. The spectra 214 and 216 fora given sub-band are acquired by a signal analyser 220. The signalanalyser 220 records the nature of the input spectrum based upon thespectrum 216. A vector subtraction step within the signal analyser 220performs prescaling and vector difference operations similar to thescaling operations involving scaler 138 and subtractor 130 in FIG. 1.The input power and distortion vector error data in each sub-band can beused by the signal analysers 220 to distinguish and quantify distortionerrors arising from different orders of intermodulation distortion andphase distortion in the amplifier 110.

Based upon the pattern of distortion error vectors in each sub-band, thesignal analyser 220 can determine which order of amplifiernon-linearity, for example, third order, fifth order, etc., isresponsible for a given measured distortion product. Consider, forexample, an amplifier amplifying multiple modulated carriers. Todistinguish and quantify the amount of third order distortion present inthe amplifier output signal 118, the signal analyser 220 first examinesthe spectrum of the input signal 216 in each sub-band to determine thefrequencies and powers of the incoming carrier or carriers. Using thisinformation, the signal analyser 220 computes a set of sub-bands andfrequencies where it expects to find significant third order distortionproducts. The signal analyser 220 then examines the sub-bands andfrequencies in the distortion vector error data where it expects to findthird order distortion products. The signal analyser 220 then collectsthe amounts of third order distortion energy found in the expectedsub-bards and frequencies to quantify the third order distortion presentin the amplifier output signal 118. This measure of third orderdistortion is then output from the signal analyser 220 and can be usedto control an external amplifier linearisation means. Similarly, thesignal analyser 220 can determine where it expects to find fifth order,etc., distortion due to the amplifier input signal 112.

It will be appreciated that scheme 200 can be used to build up aspectrum of the residual distortion across the entire frequency band ofinput signal 112 by calculating the residual distortion spectra atdifferent sub-bands by varying the frequency of local oscillator signalproduced by synthesiser 128. As with detection scheme 100, in detectionscheme 200, the series of sub-bands dictated by local oscillator signal128 may be overlapping, abutting or providing coverage of only parts ofthe frequency band of input signal 112. It may be desirable to movethrough the series of sub-bands in a sequence which is pseudo random. Inthis way, each sub-band is processed at a pseudo randomised time. Thisavoids any interactive effects between the sequence of sub-bands sampledand the temporal characteristics of the input signal 112. In general, itis also possible for the sub-bands to be of unequal or adjustablefrequency width to allow different parts of the frequency band of theinput signal 112 to be covered with differing resolutions. This mayassist the optimisation of distortion suppression and convergence speed(of a distortion reduction mechanism) for certain regions of thefrequency band of the input signal 112 where linearisation is consideredmore important.

The detection schemes described herein may also be rendered adaptive by,for example, arranging that the sub-band bandwidths and/or sub-bandsequences adapt to the occupancy of the frequency band of the inputsignal 112. An adaptive bandwidth scheme is facilitated by digitalimplementations of the filtering operations within the DCFD functions.This allows the filtering operations to have an adjustable bandwidth.The resolutions and integration intervals of the FFT operations can alsobe adjusted to adapt the frequency resolution within a sub-band or tovary the data collection time at each sub-band frequency.

An adaptive distortion detection scheme would advantageously allow adistortion suppression mechanism to achieve better distortionsuppression and convergence speed. Adaption would allow distortiondetection sensitivity and performance to be concentrated exactly whereit is needed within the frequency band of the input signal 112. Thus,enhanced distortion suppression performance would be obtained over theportions of the frequency band which are of interest at the expense ofother parts of the frequency band where distortion suppression is notrequired (perhaps because the latter parts of the frequency band are notin use at a given time). Normally, it is observed that a linearisedamplifier provides variable linearity across the band of interest. Theabove described adaptive techniques offer a way of steering thefrequencies where better linearity can be obtained from the combinationof the non-linear amplifier and the distortion suppression mechanism tomatch the frequencies occupied in input signal 112 at any one time.Therefore, the distortion suppression can be optimised to have the mosteffect at the frequencies of the strongest signals where it is mostneeded.

In a variation of detection schemes 100 and 200, the phase andamplitude, or inphase and quadrature (IQ) elements of the downconvertedsub-band signals 134 and 136 can be detected and used to control bothamplitude and phase (or both I and Q parts) of, for example, a phase andamplitude predistorter.

1. Apparatus for detecting distortion in an output signal which signalhandling equipment produces in response to an input signal, theapparatus comprising: an isolator adapted to isolate an input frequencysub-band of the input signal and an output frequency sub-band of theoutput signal; a comparator adapted to compare a measure of the inputfrequency sub-band to a measure of the output frequency sub-band toassess the output signal for distortion; and a controller adapted tochange the frequency of the input frequency sub-band and the frequencyof the output frequency sub-band to assess the distortion in the outputsignal at a plurality of different frequency sub-bands.
 2. The inventionof claim 1, wherein the isolator comprises: a synthesizer adapted togenerate a local oscillator (LO) signal; an input down conversion,filtering, and digitization (DCFD) unit adapted to isolate the inputfrequency sub-band of the input signal based on the LO signal; and anoutput DCFD unit adapted to isolate the output frequency sub-band of theoutput signal based on the LO signal, wherein: the controller is adaptedto control frequency of the LO signal to change the frequency of theinput frequency sub-band and the frequency of the output frequencysub-band.
 3. The invention of claim 2, wherein the controller is adaptedto control the frequency of the LO signal in a sequence which is not infrequency order in order to isolate the frequency sub-bands in adiscontinuous manner.
 4. The invention of claim 3, wherein thecontroller is adapted such that the frequency sub-bands are isolated ina pseudo-random manner.
 5. The invention of claim 3, wherein thecontroller is adapted such that the frequency sub-bands are isolated inan adaptive manner.
 6. The invention of claim 5, wherein the adaptivemanner concentrates on frequency sub-bands having stronger inputsignals.
 7. The invention of claim 1, wherein the measures of the inputand output frequency sub-bands are powers of the sub-bands.
 8. Theinvention of claim 1, wherein: the isolator is adapted to transform theinput and output frequency sub-bands into a frequency domain; and themeasures of the input and output frequency sub-bands are based onfrequency spectra of the sub-bands in the frequency domain.
 9. Theinvention of claim 8, wherein the comparator is adapted to performvector subtraction on the input and output frequency spectra to assessthe distortion in the output signal.
 10. The invention of claim 1,wherein the comparator is adapted to correlate (i) a residual signalgenerated from the measures of the input and output frequency sub-bandswith (ii) a reference distortion signal.
 11. The invention of claim 10,wherein the reference distortion signal is derived by multiplying theinput frequency sub-band by itself repeatedly.
 12. The invention ofclaim 1, wherein the distortion is one or both of phase distortion andamplitude distortion.
 13. The invention of claim 1, wherein at least twofrequency sub-bands have different frequency widths.
 14. A method fordetecting distortion in an output signal which signal handling equipmentproduces in response to an input signal, the method comprising:isolating an input frequency sub-band of the input signal and an outputfrequency sub-band of the output signal; comparing a measure of theinput frequency sub-band to a measure of the output frequency sub-bandto assess the output signal for distortion; and changing the frequencyof the input frequency sub-band and the frequency of the outputfrequency sub-band to assess the distortion in the output signal at aplurality of different frequency sub-bands.
 15. The invention of claim14, wherein isolating the frequency sub-bands comprises: generating alocal oscillator (LO) signal; isolating the input frequency sub-band ofthe input signal based on the LO signal; and isolating the outputfrequency sub-band of the output signal based on the LO signal, wherein:the frequency of the LO signal is controlled to change frequency of theinput frequency sub-band and the frequency of the output frequencysub-band.
 16. The invention of claim 15, wherein the frequency of the LOsignal is controlled in a sequence which is not in frequency order inorder to isolate the frequency sub-bands in a discontinuous manner. 17.The invention of claim 16, wherein the frequency sub-bands are isolatedin a pseudo-random manner.
 18. The invention of claim 16, wherein thefrequency sub-bands are isolated in an adaptive manner.
 19. Theinvention of claim 18, wherein the adaptive manner concentrates onfrequency sub-bands having stronger input signals.
 20. The invention ofclaim 14, wherein the measures of the input and output frequencysub-bands are powers of the sub-bands.
 21. The invention of claim 14,wherein: isolating the frequency sub-bands comprises transforming theinput and output frequency sub-bands into a frequency domain; and themeasures of the input and output frequency sub-bands are based onfrequency spectra of the sub-bands in the frequency domain.
 22. Theinvention of claim 21, wherein vector subtraction is performed on theinput and output frequency spectra to assess the distortion in theoutput signal.
 23. The invention of claim 14, wherein a residual signalgenerated from the measures of the input and output frequency sub-bandsis correlated with a reference distortion signal.
 24. The invention ofclaim 23, wherein the reference distortion signal is derived bymultiplying the input frequency sub-band by itself repeatedly.
 25. Theinvention of claim 14, wherein the distortion is one or both of phasedistortion and amplitude distortion.
 26. The invention of claim 14,wherein at least two frequency sub-bands have different frequencywidths.